Microenvironmental sensors
FIELD OF THE INVENTION 
     The invention relates to microenvironmental sensors, that is sensors for use in sensing environments of dimensions of the order of 10 -3  -10 -7  meters. 
     BACKGROUND OF THE INVENTION 
     A microenvironmental sensor may be in the form of a probe carrying a single sensing interface or an array of such interfaces, each interface being of micro dimensions and mounted on a substrate of the probe, the substrate having micro transverse dimensions. Typically the interface is carried on a shank portion of the probe. Such a probe is used in conjunction with signal processing circuitry. 
     A typical application of microenvironmental sensors is in physiological work in which it is frequently necessary to use biosensors of extremely small physical dimensions. For example, it may be desired to sense neurological changes of electrical potential by implanting a sensing interface in a single nerve cell with minimal damage to and effect on the surrounding tissue and environment. 
     Conventional microenvironmental sensors are large, they have substrates of more than 100μ wide, 25-30μ thick and sensing interfaces of dimensions more than about 10μ. These suffer from the disadvantages of disturbing and often irreversibly modifying the microenvironment into which they are placed, especially of the microenvironment around the sensing interfaces. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to overcome this problem by providing a method of making a microenvironmental sensor of very small dimensions. 
     According to the invention there is provided a method of making a microenvironmental sensor from a substrate having a crystalline structure, the method comprising applying sensor means to the substrate surface, anisotropically etching around the sensor means in accordance with the crystal orientation, to produce a required thickness of sensor, and releasing the sensor from the substrate. 
     The present invention employs substrates of crystalline materials that can be etched by an etchant compatible with the material and acting anisotropically so as to etch the crystal preferentially in a certain crystallographic direction. 
     Sensors made in accordance with this invention can be made precisely to dimensions of the order of 10 -3  -10 -7  meters. They may have substrates at least a portion of which is of reduced width, typically less than 100μ, and reduced thickness, typically less than 20μ, and sensing interfaces thereon of reduced size, typically less than 5μ. This is especially advantageous for sensors to be used in microenvironments, especially delicate situations in which damage to and effect on the surroundings of the sensor, especially the sensing interface, must be minimised, such as in testing a physiological environment such as human, animal or plant tissue and cells. Other environments which may be tested are soils or metals. 
     The preferred substrate material is silicon. Other materials which may be used are for example other group 4 elements such as germanium. 
     The sensor of the invention may be fixed to a carrier which incorporates any required signal processing circuitry. Alternatively, the sensor itself may include signal processing circuitry. 
     The sensor means preferably comprises conductive means such as metal or polysilicon. Preferred metals are tantalum, titanium, gold, silver, platinum, iridium, aluminium, nickel, rhodium. The conductive means may be insulated from the substrate, preferably by silicon dioxide and/or silicon nitride. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A microenvironmental sensor according to the invention will be described in detail with reference to the following drawings, in which: 
     FIG. 1 is a plan view in outline of a sensor produced according to the invention; 
     FIG. 2 is a side view of the shank of the sensor; 
     FIG. 3 is a surface view of the sensor showing an array of sensor interfaces, conductors and contact pads before the application of a final insulating and protective layer; 
     FIG. 4 shows a probe mounted in a carrier with three four channel amplifiers; and 
     FIGS. 5a-5d comprise diagrams showing successive stages in the making of a probe according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     Referring first to FIG. 1, a sensor consists of a silicon base forming an approximately square body of about 2 mm side from which projects a shank about 700 micrometers in length, 100 micrometers wide and 15 micrometers in depth, tapering to a sharp edge at its extremity as is shown in the side view of FIG. 2. 
     FIG. 3 shows the arrangement of conductors in the sensor. The shank 2 is provided along one edge with a row of twelve sensor interfaces 3 at 50 μm intervals and each interface is connected by a separate conductor 4 to a respective contact pad 5. Each interface is of dimensions in the range 1-2μ. The view in FIG. 3 is taken before the final stage of manufacture which comprises coating the conductors and the surface of the body with an insulating and protective layer leaving exposed only the sensor interfaces 3 at the very edge of the shank of the sensor and a window over each of the contact areas 5 indicated by the dotted outlines 6. 
     When the manufacture of the sensor has been thus completed it is fitted face down into a carrier as shown in FIG. 4, the carrier making contact with the twelve contact pads and connecting them to the inputs of three four-channel amplifiers f mounted in the carrier. The overall dimensions of the carrier are approximately 4.5 mm long, 2.3 mm wide and 0.25 mm thick. The carrier may incorporate any required signal processing circuitry. 
     The sensor is made from a silicon wafer as follows. 
     A pattern of conductors corresponding to a sensor corresponding to the arrangement shown in FIG. 3, is formed on one surface of a silicon wafer, which may be a single crystal, by use of known techniques employed in the manufacture of integrated circuits. For example, the conductors may be laid down by vacuum deposition, sputtering or selective etching of a uniformly applied metal layer. The conductors are insulated from the silicon substrate, for example by silicon dioxide. The metal of the sensor interface must not react with the constituents of living cells. A preferred metal is gold, which is biocompatible, chemically stable and not prone to oxidation. The gold is adhered to the conductor via a layer of chrome. The conductors of the sensor are finally protected by an insulating and protective layer, as already described with reference to FIG. 3. 
     When the conductors and insulators of the sensor have been formed in this way or prior to this, a layer of photoresist is applied to the surface of the silicon wafer and exposed through a mask so as to define the outlines of the sensor as shown in FIG. 1. Then the unexposed portions of the photoresist are dissolved away. 
     Further lithography is used to mask the conductors and the unmasked surface of the wafer is etched with an etchant which has a strongly preferred direction of action along a particular crystallographic axis. Such a highly anisotropic etchant may consist, for example, of ethylene diamine, pyrocatechol and water. By the use of this etchant the unmasked silicon surrounding the probes is etched away, leaving the sensors upstanding as mesas above the surrounding surface. 
     The photoresist is then removed with a suitable solvent from the surface of the upstanding sensor and this surface of the wafer is attached with black wax to a teflon paddle. The wafer mounted on its paddle is then immersed into an isotropic further etchant solution which etches away the silicon from the rear until the sensor is separated from the wafer, remaining fixed by the wax to the teflon paddle. In the final stage of the manufacture, the sensor is removed by dissolving the wax with an appropriate solvent. 
     The final etching stages are shown diagrammatically in FIGS. 5a-5d. In FIG. 5a the silicon wafer 7 carries the sensors, each of which is protected by a layer of hardened photoresist 8. FIG. 5b shows the same wafer section after it has been etched from the front surface by the anisotropic etchant, leaving the sensor upstanding as a mesa with its protective layer of photoresist, above the general level of the front surface of the wafer. FIG. 5c shows the section of the wafer with the resist removed, and FIG. 5d shows the separated sensor after the wafer has been eteched away from the rear surface by the final etch, which is isotropic. 
     More than one sensor may be made from a single wafer as shown in FIGS. 5a-5d. 
     The sensing interfaces employed in this invention may be in the form of simple electrodes or they may incorporate various interfacing materials to produce various functions. They may comprise thermocouples of dissimilar metals for example.